The present invention relates to a semiconductor integrated circuit for controlling and driving light emitting devices and an operation method thereof, and particularly to a technology effective in generating the optimum drive voltage even with respect to fluctuations in temperature and variations in the characteristics of light emitting devices, etc, and reducing power consumption of a system.
A light emitting diode (LED) is a semiconductor device which applies a voltage in its forward direction to thereby emit light. The principle of light emission or luminescence thereof utilizes an electro-luminescence (EL) effect. Organic EL (OLEDs: Organic light-emitting diodes) are also included in light emitting diodes.
The light emitting diode utilizes a PN junction structure of a semiconductor. The luminescence aims to directly convert energy held in electrons into light energy and requires no intervention of heat and kinetic energy. With the voltage applied in the forward direction, electrons and positive holes respectively flow into a conduction band and a valence band and are recombined beyond a band gap in the neighborhood of a PN junction portion. Upon their recombination, energy equivalent to an approximately forbidden bandwidth is emitted as photons or light. The wavelength of emitted light is determined by the bandgap of a semiconductor material and basically takes the form of single-color luminescence. Light emitting diodes of various neutral colors such as a white color, incandescent, etc. have also been produced by applying fluorescent coating onto the surfaces of the light emitting diodes that emit blue, purple and ultraviolet rays.
For the luminescence of each light emitting diode, it is necessary to apply a supply voltage greater than a forward voltage VF for a PN junction between its anode and cathode. The amount of light emitted by the light emitting diode depends on the forward amount of current. The forward voltage VF of the light emitting diode varies according to the luminescent color. The forward voltage VF is 1.4V or so at infrared light. The forward voltage VF is 2.1V or so at the red, orange, yellow and green colors. The forward voltage VF is 3.5V or so at the white and blue colors. The forward voltage VF ranges from about 4.5V to about 6V at the ultraviolet rays.
Light emitting diodes having various luminescent colors can be manufactured by using the following semiconductor materials in association with the luminescent colors of the light emitting diodes.
Aluminum gallium arsenide (AlGaAs)—infrared·red
Gallium arsenide phosphide (GaAsP)—red·orange·yellow
Indium gallium nitride (InGaN)/Gallium nitride (GaN)/Aluminum gallium nitride (AlGaN)—(orange·yellow·) green·blue·purple·ultraviolet
Gallium phosphide (GaP)—red·yellow·green
Zinc selenide (ZnSe)—green·blue
Aluminum indium gallium phosphide (AlGaInP)—orange·golden yellow·yellow·green
On the other hand, the white light is light achieved by a continuous spectrum throughout the visible rays, whereas the light emitting diode emits only wavelengths in a specific range. For this reason, a white light emitting diode cannot be materialized in the original sense. There has, however, been proposed a pseudo-white light emitting diode using the nature of human eyes. This light emitting diode utilizes the property that a mixture of three primary colors and a mixture of two colors placed in a complementary-color relationship are also visible to the human eyes as the white color. This is used as a substitute for the white light.
For example, a fluorescent-material type white light emitting diode has a structure in which a chip for a light emitting diode is coated with a luminescent material in the form of a system in which a blue light emitting diode or a light emitting diode having a wavelength shorter than it and a fluorescent material are combined together. When it is lit up, a mixture of light by fluorescence and light of the light emitting diode, which has penetrated through the luminescent material, is obtained. Thus, the white light can be obtained by adjusting a fluorescence wavelength, the thickness of the fluorescent material, etc.
A blue/yellow-system pseudo white light emitting diode is a mainstream of the current white light emitting diode. A white light emitting diode that is very light in visual form is achieved by combining a fluorescent material fluorescent to the yellow having a wavelength high in visibility, and a blue light emitting diode.
Since the light emitting diode (LED) is low in power consumption, long-lived and small in size, it has been used in a vast number of electronic equipment. As concrete applications, the light emitting diode is applied to a traffic light, directional signs at traffic-related stations/airports, a large-sized vision, an advertising display or the like, backlights for a liquid crystal television and a notebook personal computer, an LED display, LED illumination for interior/outdoor lighting, etc. An expansion of its application range is moving ahead.
A driver for driving an LED array of white light emitting diodes, which is used as a backlight source for a large-sized liquid-crystal display, has recently been made in public from each of semiconductor makers. This driver is capable of driving in parallel, a plurality of LED strings. At each of the strings, a plurality of light emitting diodes (LEDs) can be coupled in series.
The following patent document 1 has described that in a light emitting device driving device capable of driving a plurality of light emitting device strings including a plurality of light emitting devices therein, constant current driving is performed regardless of variations in the characteristics of the light emitting devices to reduce power losses. A high voltage generated from a step-up switching power supply circuit is supplied to a plurality of anodes of the top-stage light emitting devices included in the light emitting device strings in common. A plurality of cathodes of the bottom-stage light emitting devices included in the light emitting device strings are respectively driven by a plurality of constant current drivers. Further, a plurality of voltages of the cathodes of the bottom-stage light emitting devices, i.e., a plurality of voltages applied to the constant current drivers are supplied to a selection circuit. The selection circuit selects the minimum voltage from the voltages and feeds it back to a control circuit as a detection voltage. Since the control circuit generates a control signal in such a manner that the detection voltage becomes equal to a reference voltage, the step-up switching power supply circuit controls the magnitude of a high voltage in response to the control signal, so that the detection voltage becomes equal to the reference voltage. The value of the reference voltage is set in such a manner that each of transistors of the constant current drivers is operated in an active region having a margin slightly more than the boundary between the active region and a saturated region to allow a constant current to flow through the transistor with reliability. As a result, since the high voltage of the step-up switching power supply circuit is automatically controlled in such a manner that the minimum voltage becomes equal to the reference voltage, it is possible to sufficiently emit light from each LED device and reduce losses of the constant current drivers, even though variations in the characteristics of the LED devices take place.
The following non-patent document 1 has described a high-efficiency driver having white light emitting diodes designed for a large-sized liquid crystal display with a LED array of the white light emitting diodes as a light source. The LED array is capable of configuring in parallel up to 8 strings having 10 series-coupled LEDs per string. An output voltage generated from a step-up controller comprised of an inductor, a power MOS transistor, a rectifying diode and a capacitor is supplied to a plurality of anodes of the top-stage LEDs of the eight strings in common. A plurality of cathodes of the bottom-stage LEDs of the eight strings are respectively driven by a plurality of current sources. Each of the current sources is comprised of a differential amplifier, an N channel MOS transistor and a resistor. The step-up controller automatically selects the minimum voltage of a plurality of feedback voltages corresponding to drive voltages of the current sources to thereby adjust an output voltage.